Zooplankton of the open Baltic: Extended Atlas - IOW

filter feeders. Cyclopoida are also planktonic crustaceans but very often they
inhabit near-bottom biotopes; they are generally micro-predators that feed on
small invertebrates and even fish larvae but also consume algae.
Harpacticoida are mainly meiobenthic or epibenthic grazers, they occur in
plankton only sporadically, being washed out from their bottom habitats by
strong water movements. In general Harpacticoida are only temporarily in
plankton, although these crustaceans are often found in zooplankton samples
collected in the shallow estuarine waters.
Copepods are food to many predators, mainly planktivorous fish. The
choice of a copepod as a prey is a function of its size, morphology, motion
(angle, speed, escape ability) and pigmentation. The coloured species are
more vulnerable to predation than pale or transparent ones. Presence of fish
can influence physiological parameters and population dynamics of copepods.
To limit predation, some copepods can retreat to habitats devoid of the
predator, perform vertical migrations, form swarms, or enter into dormancy
(Dussart & Defaye, 2001).
Copepods have different tolerance to salinity; the presence or absence
of some species allows deductions on the physical-chemical characteristics or
the degree of pollution of the environment (Dussart & Defaye, 2001).
However, the important role of copepods as biological indicators
cannot be assessed unless the copepod species identification is properly
fulfilled. Taxonomic differentiation of copepods is based mainly on external
morphology of mature females and males. Species identification of copepods
is an important though tedious procedure. Shape, colour and size of the body,
relative size of the appendages (particularly the length of antennules relative
to the cephalosome or the urosome) and other measurements are noted. After
general observations drawings of the whole animal should be made.
For cleaning the crustacean and making its body more transparent, the
animal must be kept in a drop of concentrated lactic acid (CH 3 CHOHCOOH)
for a time from 1 h up to overnight, depending on the size of the crustacean.
Sometimes it is possible to recognize the copepod species without dissection
(Alekseev, 2002; Telesh & Heerkloss, 2004). However, in most cases species
identification of copepods requires not only examination of the whole
crustacean under the microscope but also a dissection and mounting of
relevant structures. For more details of this procedure see Downing and Rigler
(1984), Huys and Baxshall (1991), ICES (2000), Dussart and Defaye (2001),
Alekseev (2002).
Copepods can be of different shape: elongated, fusiform, or cylindrical.
General schemes of body morphology of cyclopoid and calanoid copepods are
presented in Figures 5.1.11 and 5.1.12; schematic drawings of their nauplia
and copepodites are given in Figures 5.1.13 – 5.1.15.
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